Lymphomas of the B lymphocyte lineage occur in high frequency as spontaneous neoplasms of NFS mice congenic for ecotropic murine leukemia virus induction loci of AKR and C58 mice. These lymphomas have been transplanted and grown in vitro clearly defining them as transformed cells. The cells are, histologically, lymphoblasts, follicular center cells, or immunoblasts and bear surface markers characteristic of normal B cells-sIg+, I-A+, Ly-5 (B220)+, ThB+, Lyb-2+, Ly-17+ or pre-B cells-sIg-, I-A-, Ly-5 (B220-), ThB-, Lyb-2+, Ly-17+. Mice with these tumors should prove to be valuable models for studying B cell neoplasia in man.